Organic EL devices which utilize organic substances are expected to be useful for application as an inexpensive full color display device of the solid light emission type having a great size and various developments on the organic EL devices are being conducted. In general, an organic EL device has a construction comprising a light emitting layer and a pair of electrodes disposed at both sides of the light emitting layer.
The light emission of the organic EL device is a phenomenon in which, when an electric field is applied between the two electrodes, electrons are injected from the cathode side and holes are injected from the anode side, the electrons are recombined with the holes in the light emitting layer to form an excited state, and energy generated when the excited state returns to the ground state is emitted as light.
As the light emitting material, chelate complexes such as tris(8 quinolinolato) aluminum, coumarine derivatives, tetraphenylbutadiene derivatives, bisstyrylarylene derivatives and oxadiazole derivatives are known. It has been reported that these light emitting materials emit light in the visible region of blue to red and it is expected that color display devices can be obtained by using these light emitting materials (for example, Japanese Patent Application Laid-Open Nos. Heisei 8(1996)239655, and Heisei 7(1995)438561).
Although the practical use of displays using organic EL devices recently started, the full color display device is still under development. In particular, an organic EL device which emits bluish light with excellent purity of color and efficiency of light emission has been desired.
As the device as the attempt to satisfy the above desire, for example, a device using a phenylanthracene derivative as the material emitting blue light is disclosed in Japanese Patent Application Laid-Open No. Heisei 8(1996)-12600. The phenylanthracene derivative is used as the material emitting blue light and, in general, used as a laminate composed of a layer of the material emitting blue light and a layer of a complex of tris(8-quinolinolato)aluminum (Alq). However, the efficiency of light emission, the life and the purity of blue light are insufficient for the practical application. In Japanese Patent Application Laid-Open No. 2001-288462, a device emitting blue light in which an amine-based aromatic compound is loused for the light emitting layer is disclosed. However, the efficiency of light emission of this device is as insufficient as 2 to 4 cd/A. In Japanese Patent Application Laid-Open No. 2001160489, a device in which an azafluoranthene compound is added to the light emitting layer is disclosed. However, this device emits light of yellow to green and cannot emit blue light having a sufficiently high purity of color.